The LL.M. is the Master at Law degree. The JSD or SJD, depending on which school attended, is the Doctor of Juridical Science. It is the highest degree offered in the legal profession. Z
No, most LL.Ms dont aspire to obtain the JSD. In fact, less than 5% of all LL.M's shoot for the JSD.
As far as lar firms are concerned, prestige and qualifications will determine salary. If you have a LL.M from Columbia, which is considered as having the best LL.M. program, or Harvard, NYU or Yale, your LL.M should make a difference in your salary. Also, if you earned a LL.M from a less prestigious school in a complex field like tax or IP, then it should make a difference.
LL.Ms are mostly useful for future law professors, specialty law practitioners, and foreign trained lawyers.